[#615] Use UNSIGNED_PAYLOAD to check sign

Use `UNSIGNED_PAYLOAD` to check signature if x-amz-content-sha256 isn't provided as signed header

https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html

" You include the literal string UNSIGNED-PAYLOAD when constructing a canonical request, and set the same value as the x-amz-content-sha256 header value when sending the request to Amazon S3"

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/auth/presign_test.go

@@ -77,8 +77,7 @@ func TestCheckSign(t *testing.T) {
 		Lifetime: 10 * time.Minute,
 		SignTime: time.Now().UTC(),
 		Headers: map[string]string{
-			ContentTypeHdr:   "text/plain",
-			AmzContentSHA256: UnsignedPayload,
+			ContentTypeHdr: "text/plain",
 		},
 	}